How to make free voice calls over Wi-Fi with Facebook Messenger for iPhone

If you're in the U.S. or Canada, Facebook Messenger for iPhone now lets you place Skype-like VoIP (voice over IP) audio calls over Wi-Fi to any of your friends who also happen to be in the U.S. or Canada. There's no FaceTime-style video calls yet, and no international calls, and the quality doesn't yet sound as good as Skype, but if you live in Facebook Messenger, it can be a convenient alternative.

The only problem is, Facebook hasn't exactly made the calling process obvious. And in some cases, it's downright cumbersome. Here's how you do it...